Process for high quality plasma arc and laser cutting of stainless steel and aluminum

ABSTRACT

Plasma arc or laser cutting uses a mix of reactive and reducing gas flows to cut sheets of stainless steel, aluminum and other non-ferrous metals. The reducing gas flow to the cut varies as a percentage of the total gas flow to maintain a reducing atmosphere down through the cut, but to leave a predominantly oxidizing atmosphere at the intersection of the cut and the bottom surface of the sheet being cut. In plasma arc cutting these flows can also be characterized as either a plasma gas flow, one that forms the arc, or a shield gas flow that surrounds the arc. The reactive gas is preferably a flow of air, oxygen, nitrogen, carbon dioxide or a combination of these gases. The reducing gas is preferably hydrogen, hydrogen 35, methane, or a mixture of these gases. For aluminum, the reactive gas is preferably air or nitrogen and the reducing gas is preferably methane or a mixture of methane and air. In laser cutting the reducing gases such as methane can be used by mixing them with reactive assist gases.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Ser. No. 07/989,183filed Dec. 11, 1992.

BACKGROUND OF THE INVENTION

This invention relates in general to plasma arc and laser cutting ofsheet metals. More specifically, it relates to a mixture of type andproportion of gases forming and/or shielding the arc or laser beam thatyield very clean, shiny, and dross-free cuts in stainless steel,aluminum and other non-ferrous metals.

Plasma arc cutting of sheet metals is now used widely. However,heretofore for stainless steel and non-ferrous metals such as aluminumit has not been possible to produce a clean cut, one where there is ashiny kerf that is free of oxides or nitrides of the metal being cut,which is also free of bottom dross.

The plasma arc is a jet of an ionized gas. While many gases can be usedto form the arc, the gas selected is usually specific to the metal beingcut. For example, to cut stainless steel, it is most common to use air,nitrogen, or a mixture of argon and hydrogen.

Nitrogen and air leave no bottom dross, but the cut quality is poor. Thesides of the kerf have oxide or nitride inclusions and they undergo achange in metallurgical structure. In order to well at this cut, or toobtain an acceptable appearance, it is necessary to grind or wire-brushthe cut sides.

It is also known that using argon-hydrogen as the plasma gas to cutstainless. While these cuts are metallurgically "sheen", that is, shinyand clean, but at least for cuts in thin sheets, argon-hydrogen leaves abottom dross that is unusually difficult to remove. Sheeny, dross-freecuts are possible with argon-hydrogen for sheets with a thickness inexcess of about 0.5 inch (12.7 mm) using a 200 ampere torch and inexcess of about 0.25 inch (6.4 mm) using a 100 ampere torch. No plasmacutting technique has been found that produces sheeny kerfs withoutdross when cutting aluminum, regardless of its thickness.

It is also well known to use shield gases, typically a secondary gasflow through the torch that is independent of the plasma gas flow andsurrounds the arc, whether by impinging on it as it exits the torch ordownstream, near or at the workpiece. Shield gases can serve a varietyof functions, such as cooling, isolation of the cutting action in thekerf from the atmosphere, and the protection of the torch againstupwardly splatterd molten metal. Plasma and shield gases are used, forexample, in the plasma arc cutting torches sold by Hypertherm, Inc. ofHanover, N.H. under its trade designations MAX™200, MAX™100, MAX™100Dand HD1070. The numbers 200, 100 and 70 denote current ratings for thesetorches. None of the known torches using shield gases have demonstratedany ability to improve on the cut quality of known nitrogen, air andargon-hydrogen cutting when used on stainless steel and non-ferrousmetals such as aluminum.

Laser cutting has suffered from similar cut quality problems when usedto cut stainless steel and non-ferrous metals. The oxygen and nitrogenassist gases form oxides and nitrides in the kerf. Good cut quality canbe obtained using helium, argon or other non-reactive gases, but cuttingwith these gases is very slow, the gas must be at high pressures, andpreferably it is highly pure, and therefore more costly.

It is therefore a principal object of the present invention to provide aplasma arc ,and or laser cutting process that can cut stainless steel,aluminum and other non-ferrous metals at commercially acceptable speedswith an extremely high cut quality.

A further principal object is to provide a cutting process that isadaptable to different metals and different torches, including highdensity plasma arc torches, and plasma torches using only a plasma gasor ones using plasma and shield gases.

Another object is to provide a cutting process with the foregoingadvantages even when used on thin sheets of the metal.

Still another object is to provide all of the foregoing advantages usingknown equipment and operating materials and at a favorable cost.

SUMMARY OF THE INVENTION

At least one gas flow to a cutting torch constitutes or contains as acomponent of a mixed flow of gases a reducing gas. The gas flows alsoinclude a gas that reacts with the metal. The flow ratio of the reducinggas flow to the total gas flow to the cut, whether introduced as aplasma and/or shield gas to a plasma arc torch, or as an assist gas inlaser cutting, is controlled so that the reducing gas is completelydissipated in the kerf. As a result, the reducing gas has a negligiblysmall concentration at region defined by the kerf and the bottom surfaceof metal workpiece. Stated conversely, the atmosphere at the bottomsurface is predominantly oxidizing. The gas selection and control of thereducing gas ratio can be defined functionally as ones which provide areducing atmosphere that extends through the kerf, from the top to thebottom surfaces of the workpiece, but which also produce an oxidizingatmosphere at the bottom surface. The ratio which yields this resultvaries empirically with the type of metal, the type and power of thetorch, the type of gases being used, and the thickness of the workpiece.For a given application, the ratio varies with the thickness. Thisprocess produces high quality cuts in stainless steel, aluminum, andother non-ferrous metals. The cuts are sheeny and free of bottom dross.

In plasma arc cutting, while this mixture of gases can be formed solelyin a plasma gas, the gases are preferably introduced as plasma andshield gases. The reactive and reducing gases can appear, solely or inmixture, as either one of, or both of, these gas flows.

To cut stainless steel with a high definition plasma arc torch at lowpower, the plasma gas is preferably air or nitrogen flowing typically at40 scfh (standard cubic feet per hour) for low to medium powerapplications. With nitrogen as the plasma gas, the shield gas can bemethane or methane and air. The ratio of the methane flow rate to airflow rate ranges from about 5% to 25% depending on the thickness of theworkpiece. A typical shield gas flow rate is in the range of 20 to 60scfh, depending on the thickness. For high definition plasma arc cuttingof aluminum, the plasma gas is again air or nitrogen with methane as ashield gas. With a nitrogen plasma gas, the methane can be mixed withair, again in varying ratios to accommodate different thicknesses.

Plasma gases for a standard plasma arc torch can include hydrogen,hydrogen 35 mixed with nitrogen, and a mixture of hydrogen and nitrogen,and air. Shield gases include nitrogen and carbon dioxide. Nitrogen isthe preferred shield gas with either the hydrogen 35 and nitrogenmixture or the hydrogen-nitrogen mixture as the plasma gas.

For stainless and aluminum plasma arc cutting, the reactive gas ispreferably nitrogen, air, other mixtures of oxygen and nitrogen otherthan air. Reducing gases can include hydrogen, hydrogen 35, methane, andother flammable hydrocarbon gases known to combine with oxygen. Thereducing gas preferably constitutes between 2% and 50% of the total gasflow --plasma gas and shield gas, if any--depending on the thickness ofthe workpiece, other parameters being constant.

For laser cutting, assist gas flows using hydrogen or a hydrogen-bearinggas suck as methane as the reducing gas produce the improved cut qualityof this invention. The assist gas flow rate, or where the assist gas isa mixture of gases, the ratio of the reducing gas to total assist gasflow, is varied to produce a predominantly reducing atmosphere withinthe kerf and a predominately; oxidizing atmosphere at the bottomsurface. As with plasma arc cutting, the ratio of reducing gas flow thetotal gas flow is between 2% and 50%, again depending on factors such asthe type and thickness of the metal forming the workpiece.

These and other features and objects of the present invention will bemore clearly understood from the following detailed description whichshould be read in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view in perspective of a prior art plasma arc,argon-hydrogen cutting of a sheet of stainless steel;

FIG. 2 is a simplified view in vertical section of a gas shield plasmaarc cutting torch operating according to the process of the presentinvention together with an associated graph showing the concentrationsof oxygen and hydrogen in the kerf as a function of the depth of thekerf;

FIG. 3 is a graph of the percentage of reducing gas in a particularplasma gas flew according to the present invention as a function of thethickness of the workpiece; and

FIG. 4 is a simplified view in vertical section of a laser cuttingdevice with an assist gas according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows prior art plasma arc cutting of a kerf 12 in a workpiece14, in this case a stainless steel plate. A plasma arc cutting torch 16of known construction produces an arc 18 that transfers from the torchto the workpiece to produce the kerf. The arc 18 is a jet of ionized gasthat conducts current to the workpiece. A DC power supply 20 isconnected in series with the torch and the workpiece. The plasma gas isan argon-hydrogen mixture, typically 35% hydrogen and 65% argon byvolume, commercially sold as hydrogen 35. A regulated, adjustable ratesupply 22 of the plasma gas is illustrated schematically. Depending onthe torch and application, the cutting torch can also receive a flow ofa shield gas from a separate regulated, adjustable flow rate supply 24.Typical torches 16 include the standard cutting torches sold byHypertherm, Inc. of Hanover, N.H. under its trade designations MAX™100,MAX™100D, and MAX™200 and its high density 70 ampere torch sold underthe trade designations "HyDefinition" and "HD1070".

This particular prior art system can cut stainless steel sheets whileproducing a clean, shiny kerf. However, as noted above, it also producesa very difficult bottom dross 26. The dross forms in two regions. Anupper region 26-1 near the kerf retains a metallic look. In a lowerregion 26-2, the dross is dark from the formation of oxides.

FIG. 2 shows a cutting system according to the present invention. As inFIG. 1, the torch shown is a known plasma arc torch such as the MAX™ andHyDefinition™ products identified above using a plasma gas flow 22a anda shield gas flow 24a. The power of the torch, as measured by itsoperating current, typically range from low power units of 15 to 50amperes, to high power units of 400 to 500 amperes. For high definitiontorches, relatively small currents, e.g. 70 amperes are typical, but ata very high current density. Typical standard torch currents for themost common thicknesses are 100 to 200 amperes.

The torch can also be a standard laser cutter 16' as shown in FIG. 4(like parts being identified with the same number, but primed in thelaser embodiment). The laser beam 28' heats the workpiece 14' at thekerf 12'. It also produces a chemical reaction between a reactive gas inthe assist flow 24a' and the metal forming a workpiece 14'. The reactivegas is typically oxygen or nitrogen. As is well known, the presence ofan active assist gas speeds the cutting action of the laser. As will bediscussed in greater detail below, according to the present inventionthe assist flow 24a' also includes a reducing gas. The light beam 28' iscoherent, and has a high energy. A lens 25 focuses the beam 28' on orwithin the workpiece 14'. The assist gas 24a' enters a housing 29through ports 29a below the lens. The assist gas flows out of the torchand into the kerf 12' in the workpiece 14' via an exit orifice 29b.

The workpiece 14 is a sheet. It can assume other forms, such as afirearm barrel, a bolt, or contoured structural member, but the cuttingof sheets, including plates, is the most common application. An "upper"surface 14a of the sheet will then be understood to be the surface ofthe workpiece opposite the plasma torch. A bottom surface 14b faces awayfrom the torch. For a sheet workpiece, the surfaces 14a and 14b aregenerally flat and parallel. The plate thickness T measured along anormal to the surfaces 14a,14b can vary from thin sheets, e.g. 1/8 inch(3.1 mm) to plates 2 inches (51 mm) thick.

A principal feature of the present invention is that the gas flow orflows from the torch to the kerf include as a constituent gas at leastone gas of a type that reacts with the metal of the workpiece, and asanother constituent gas a different type of gas that produces areduction reaction, particularly one that will react chemically in areduction reaction with reactive gases such as oxygen, or nitrogen, or amixture of the two such as air. In plasma cutting, the reactive gas andthe reducing gas can be mixed to form the plasma gas, or the shield gas,or they can be separated, one in the plasma gas flow and the other inthe shield gas flow. In laser cutting, the reacting and reducing gasesare mixed to form the single assist gas flow 24a.

A further principal feature of the present invention is that the amountof the reducing gas is carefully controlled as a portion of the totalgas flow to the kerf--the sum of the plasma and shield gases where bothare used. (Some ambient air or other gas flows may also enter the kerf,but they are usually present in insignificant amounts or aresufficiently removed from the cutting action as to be of little or nofunctional consequence.) The degree of control is conveniently expressedas the ratio of the flow rate of the reducing gas or gases to the totalgas flow rate. This ratio varies with parameters such as the type ofmetal being cut, its thickness, the type and power of the porch, and thetype or types of gas forming the plasma and shield gas flows. For agiven application, the control ratio varies mainly as a function of theplate thickness. FIG. 3 shows a typical such relationship for thecutting of stainless steel plate with a MAX™100D brand plasma arc torchwith a mixture of argon, hydrogen and nitrogen. The curve in FIG. 3snows that for this example the ratio of hydrogen to the total gas flowshould be about 3.5% for thin plates (1/8 inch), but about 32% for thickplates (1/2 inch). While the precise values will vary for eachapplication, the general form of the curve shown in FIG. 3 defines thisrelationship. In general, the ratio of the reducing gas to total gasflow that will provide the results of the present invention for bothplasma arc and laser applications falls in the range of about 2% toabout 50%. The precise value for each application can be determinedempirically by examining the cut quality for different ratios at aselected thickness, or at different thicknesses for a selected ratio.

This ratio control produces a predominantly reducing atmosphere withinthe kerf at the arc. This reflects a predominant concentration of thereducing gas extending from the upper surface 14a, substantially throughthe kerf, to a region 28 at the intersection of the kerf and the bottomsurface 14b. At the region 28 there is then predominantly oxidizingatmosphere. This is reflected in FIG. 2 in the high concentration ofreactive gas (e.g. oxygen) at the surface 14b and the negligibleconcentration of reducing gas (e.g. hydrogen). When properly controlled,it is believed that the amount of the hydrogen or other reducing gaspresent in the flow is used up in chemical reaction with the reactivegas in the kerf. This condition produces cuts in stainless steel andnon-ferrous metals of a quality that heretofore never been obtainedusing plasma arc cutting, regardless of the thickness of the workpiece.This condition also allows laser cutting with a high cut quality atspeeds heretofore unattainable, and without constraints on gas purityand pressure which have heretofore been associated with non-reactiveassist gases such as helium and argon.

While the precise mechanism(s) that produce this result are not knownwith certainty, applicants are of the opinion that the predominantlyreducing atmosphere in the kerf prevents an oxidizing reaction betweenthe molten metal being cut and reactive gases president in the kerf.(The oxidizing reaction is the one which cuts the metal, e.g. thecreation of oxides or nitrides of the metal being cut which are carriedaway by the plasma jet or the action of laser beams and associated gasflows on the material.) The reducing gas (or its ions or radicals formedin the plasma) is believed to react with the oxidizing gas (or its ionsor radicals formed in the plasma) preferentially. In the region 28, thepredominantly oxidizing atmosphere is believed to be essential tooxidize molten metal before it runs out of the bottom of the kerf toform a dross. This analysis provides a functional guide for the controlover the reducing gas portion of the total gas flow. If there is toolittle reducing gas, the kerf will not be sheeny throughout. If there istoo much reducing gas, a dross will form.

As an illustration of the process of the present invention, but not as alimitation, applicants give the following examples of this inventionwhich have been successfully practiced using Hypertherm MAX™100D andHyDefinition HD1070™ plasma arc cutting systems on stainless steel andaluminum sheets having thicknesses that varied from 1/8 inch to 5/8inch.

Using an HD1070™ system to cut stainless steel, the followingcombinations of plasma and shield gases were used successfully attypical flow rates of 40 scfh for the plasma gas and 20 to 60 scfh forthe shield gas, with the variation in shield flow rate corresponding tothe thickness of the workpiece generally as shown in FIG. 3.

                  TABLE I                                                         ______________________________________                                        (High Density, Stainless)                                                     Plasma Gas         Shield Gas                                                 ______________________________________                                        N.sub.2            CH.sub.4 (methane)                                         air                CH.sub.4                                                   N.sub.2            CH.sub.4 and air                                           air                CH.sub.4 and air                                           ______________________________________                                    

The ratio of methane to air varies from about 5:95 to 25:75 depending onthe thickness of workpiece, the total shield gas flow rate beingconstant.

Using the HD1070™ system to cut aluminum, Table II gives successfulplasma and shield gases at the flow rates given above with respect toTable I. The shield gas mix of air and methane is variable from almost100% methane to almost no methane, depending again on the thickness ofthe aluminum sheet being cut.

                  TABLE II                                                        ______________________________________                                        (Aluminum)                                                                    Plasma Gas          Shield Gas                                                ______________________________________                                        air                 CH.sub.4                                                  N.sub.2             CH.sub.4 and air                                          ______________________________________                                    

Table III gives suitable plasma and shield gases for cutting stainlesssteel with a MAX™100D plasma arc cutting system. Typical flow rates arethose given above with respect to Table I.

                  TABLE III                                                       ______________________________________                                        (Standard Arc, Stainless)                                                     Plasma Gas            Shield Gas                                              ______________________________________                                        Hydrogen 35 and N.sub.2                                                                             N.sub.2                                                 H.sub.2 and N.sub.2   N.sub.2                                                 Hydrogen 35 and N.sub.2                                                                             CO.sub.2                                                H.sub.2 and N.sub.2   CO.sub.2                                                ______________________________________                                    

The percentage of hydrogen 35 in the mixture varies from about 10% forthin sheets to about 90% for thick sheets. The percentage of H₂ in thesecond and fourth mixtures varies from about 3.5% for thin sheets toabout 35% for thick sheets.

There has been described a process which produces high quality--sheenyand dross free--cuts in stainless steel and non-ferrous metals such asaluminum using plasma arc and laser cutting. The invention can producethese results on sheets or other configurations having any of a widevariety of thicknesses using high density plasma cutting systems,standard plasma cutting systems and standard laser systems. Theinvention is also compatible with plasma cutting systems operating overa wide range of power levels and with mechanical shields and gas flowshields against upwardly splattered molten metal.

While the invention has been described with respect to its preferredembodiments, it will be understood that various modifications andvariations will occur to those skilled in the art from the foregoingdetailed description and the accompanying drawings. For example, whilethe examples use mainly nitrogen and air as the reactive gases, otherreactive gases including oxygen alone, oxygen-bearing gases, andoxygen-nitrogen mixes not in the proportion of air are contemplated.Similarly, other reducing gases, particularly hydrogen bearing gases,can be used. In particular methane is illustrative of a class offlammable gases that combine with oxygen in an exothermic reaction,although perhaps having a greater cost or producing undesirablebyproducts. Hydrazane (N₂ H₄) is one such hydrogen-bearing gas that canbe used. These and other modifications and variations that occur tothose skilled in the art are intended to fall within the scope of theappended claims.

What is claimed is:
 1. A metal cutting process for use with a plasma arccutting torch for producing a high quality kerf in stainless steel andnon-ferrous workpieces that have an upper surface adjacent the torch anda bottom surface opposite the torch, where the torch uses a total gasflow to the kerf, comprising, forming a portion of the total gas flowfrom a reducing gas, and adjusting the ratio of said reducing gas flowto said total gas flow to produce a predominantly reducing atmospheregenerally at the region defined by the bottom surface and the kerf, saidadjusting including increasing the proportion of said reducing gasintroduced into the kerf with respect to the total gas flow incoordinating with an increasing of the thickness of the workpiece. 2.The high quality cutting process of claim 1 wherein the torch is aplasma arc torch using at least a plasma gas that forms the arc and mayuse a shield gas that generally surrounds the arc at the workpiece, theplasma and shield gas flows constituting said total gas flow and whereinsaid adjusting comprises mixing at least one gas that is reactive withthe metal and at least one reducing gas.
 3. The high quality cuttingprocess of claim 2 wherein said mixing is a mixing of said plasma andsaid shield gases.
 4. The high quality cutting process of claim 2wherein said mixing is a mixing of component gases of at least one ofsaid plasma gas and said shield gas.
 5. The high quality cutting processof claim 1 further comprising the step of limiting the concentration ofthe reducing gas in the total gas flow to a value such that itsconcentration decreases to a negligible value at said region.
 6. Thehigh quality cutting process of claim 5 wherein said limiting rangesfrom about 2% to about 50% of said total gas flow.
 7. A process for highquality cutting using a plasma arc torch to cut stainless steel andnon-ferrous metal workpieces with the torch using a total gas flowthrough the torch that enters the kerf, comprising forming said totalgas flow of at least one reactive gas and at least one reducing gas, andadjusting the relative proportions of said at least one reducing and atleast one reactive gases so that said kerf is substantially sheeny andhas substantially no bottom dross, said adjusting including increasingthe proportion of said reducing gas introduced into the kerf withrespect to the total gas flow in coordination with an increasing of thethickness of the workpiece.
 8. The high quality cutting process of claim7 wherein the torch is a plasma arc torch that produces an arc in aplasma gas flow that transfers from the torch to the workpiece where itcuts a kerf through the workpiece to a bottom surface, and can include ashield gas flow that surrounds the transferred arc and the plasma gasflow and shield gas flow define the total gas flow, and wherein saidmetal is stainless steel, said reactive gas is selected from the groupconsisting of oxygen, nitrogen, carbon dioxide and mixtures of thesegases, including air, and said reducing gas is selected from the groupconsisting of hydrogen, hydrogen 35, methane, hydrazane and mixtures ofthese gases.
 9. The high quality cutting process of claim 7 wherein thetorch is a plasma arc torch that produces an arc in a plasma gas flowthat transfer from the torch to the workpiece where it cuts a kerfthrough the workpiece to a bottom surface, and can include a shield gasflow that surrounds the transferred arc and the plasma gas flow andshield gas flow define the total gas flow, and wherein said non-ferrousmetal workpiece is aluminum and said reactive gas is selected from thegroup consisting of nitrogen and air and said reducing gas is methane.10. A process for producing a high quality cut in sheets of stainlesssteel and non-ferrous metals with a plasma arc cutting torch locatedopposite an upper surface of the sheet and cuts a kerf that extends to abottom surface of the sheet and where the torch uses a total gas flowfrom the torch to the kerf, comprising, forming said plasma gas at leastin part of a reactive gas, forming said total gas flow at least in partof a reducing gas, and controlling the ratio of the reducing gas flow tothe total gas flow such that the reducing gas is consumed in the kerfleaving a negligible concentration of said reducing gas at said bottomsurface, said controlling including increasing the proportion of saidreducing gas introduced into the kerf with respect to the total gas flowin coordination with an increasing of the thickness of the workpiece.11. The process according to claim 10 wherein the torch is a plasma arctorch, that produces an arc in a plasma gas flow from the torch thattransfers to the workpiece and the torch can have a shield gas flow thatsurrounds the transferred arc, the plasma gas flow, when used without ashield gas, and the sum of the plasma gas flow and shield gas flow whenboth are used, defining said controlling includes varying said ratio gasflow within the range of about 2% to about 50% corresponding to thethickness of the workpiece.
 12. The process according to claim 10 or 11wherein said reactive gas is selected from the group consisting ofoxygen, nitrogen, carbon dioxide and mixtures of these gases, includingair, and wherein said reducing gas is selected from the group consistingof hydrogen, hydrogen 35, methane, and mixtures of these gases.
 13. Theprocess according to claim 10 or 11 where said metal is aluminum or analuminum alloy, said reactive gas is selected from the group consistingof air and nitrogen and said reducing gas is selected from the groupconsisting of methane and a mixture of air and methane.